1. Field of Invention
The present disclosure relates to Faraday cups used for measuring current in a beam of charged particles and, more particularly, to an integrated Faraday cup and optical element that can measure not only charged particle beam currents and longitudinal energy spread via retarding field energy analysis, but can also manipulate and transmit light.
2. Description of Related Art
Faraday cups may be used to measure current in a beam of charged particles. A Faraday cup may include a conducting metallic enclosure or cup that captures a charged particle beam in a vacuum. An electrical connection between the Faraday cup and a measuring instrument may relay the current to the measuring instrument. Because the charged particle beam current may be very small, a grounded picoammeter may be a suitable choice for current measurement. In addition, steps may also be taken to stop or reduce secondary emissions from distorting the current measurement. Such potentially distortive emissions include secondary electron emissions.
An optical element may comprise one or more optical elements designed to transmit or manipulate light. Faraday cups and optical elements or systems are incompatible. Faraday cups are designed to measure charged particle beam currents, and are not generally designed to transmit light or manipulate light. Unlike Faraday cups, optical elements or systems are designed to manipulate and/or transmit light. Because Faraday cups are not designed for light manipulation or transmission, optical applications may not be enabled on an axis that passes through a Faraday cup.
Unlike Faraday cups, optical elements are not usually designed to measure charged particle beam currents or transmit charged particles. Optical elements are typically manufactured using optically transparent materials which are typically insulating and, therefore, incompatible with charged particles. Accordingly, charged particle beam current measurements may not be enabled along the axis of an optical system. Optical elements are generally involved in generating, propagating, and detecting electromagnetic radiation having wavelengths within a range between the wavelengths of x-rays and microwaves.
In some situations, it may be desirable to simultaneously measure beam current and manipulate light along the same spatial axis. Such dual capabilities or functionalities may be particularly useful for certain applications, such as those that incorporate magneto-optical trap based ion sources (MOTIS). The charged particle source in a MOTIS is a laser-cooled collection of atoms called a magneto-optical trap (MOT). Charged particles, including ions and electrons, may be created in the MOTIS by photo-ionizing atoms in the MOT. These charged particles may be removed from the location of the MOT along an axis defined by an applied electrical field. Along this axis, ions and electrons may be emitted in opposite directions. Charged particle beam current measurements may be taken along this axis.
Because there is a one-to-one correspondence between the number of electrons and ions emitted in a MOTIS, the ion beam current can be determined by measuring the electron beam current. Thus, runtime monitoring of the ion beam current is possible without interrupting the operation of the ion beam. This functionality could be used to generate a control signal for feedback to enhance the stability of beam current. The axis defined by the electrical field is also very convenient for high numerical aperture optical imaging of the MOT. Also, by focusing the photo-ionization laser along this axis, the MOTIS can be operated in a “high-current” mode which may be particularly useful for creating small ion beam sources.
There is a need for a device that integrates the functionalities of a Faraday cup and optical elements by permitting the dual capabilities of beam current measurement, and light manipulation along the same spatial axis.
At times, it may be desirable to measure the energy spread or distribution of a charged particle beam. Energy spread measurements in a charged particle beam may be helpful in a number of ways. For example, energy spread measurements may assist in determining the effects of chromatic aberration of a given ion optical system.
There is a need for a device that integrates the functionalities of a Faraday cup and optical elements by permitting beam current measurement, retarding field energy analysis and light manipulation along the same spatial axis.